fs/cifs: require sha512

This got lost in commit 0fdfef9a,
which removed CONFIG_CIFS_SMB311.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Fixes: 0fdfef9a ("smb3: simplify code by removing CONFIG_CIFS_SMB311")
CC: Stable <stable@vger.kernel.org>
CC: linux-cifs@vger.kernel.org
Signed-off-by: Steve French <stfrench@microsoft.com>